In recent years, more and more light-emitting devices, such as illuminations, projectors, flashes, and headlights and tail lamps of automobiles, are using light emitting diodes (LEDs) as a light source. In such light-emitting devices, especially in devices using a narrow directional LED as a light source, a lens that collects or collimates the light emitted from the light-emission surface of the LED is used. A convex refractive lens is generally used as such a lens. Since the convex refractive lens is thick at the convex portion, it has been also proposed to adopt a Fresnel lens in order to make the lens thinner.
A lens for lighting fixtures has conventionally been proposed (see Patent Literature 1). The lens has a grating-shape refracting prism portion provided in a central part of an inner surface in the vicinity of an optical axis (i.e., a center axis of the lens), the inner surface being a light-incident surface facing a light source. The lens also has a grating-shape reflecting prism portion in a peripheral part of the grating-shape refracting prism portion. A lens member has also been proposed which is formed so that a Fresnel lens surface provided on a light-incident surface has a plurality of prisms (see Patent Literature 2). Part of incident light beams is totally reflected by a non-lens surface provided on some of the plurality of prisms and is then emitted to a light-emission surface on the opposite side of the light-incident surface. Further, an optical device has been proposed which has a refractive lens portion and a reflector portion provided on a central part centering around a center axis of a lens member (see Patent Literature 3). The reflector portion allows light beams to be incident on an inner surface portion while the light reflection surface having a paraboloidal shape totally reflects the light beams and converts the light beam into parallel beams of light.
However, in each of the above-described Fresnel lenses, when top ends of the prisms have an acute angle formed from a prism incident surface and a prism reflection surface, metal molds for the prisms that are to be filled with a resin are formed to have narrow top ends, and accordingly, a resin may not fully enter therein. In that case, the top ends of the prisms are rounded, which prevents high-accuracy light incidence and reflection at the top ends of the prisms. This causes problems of degradation in performance such as front illuminance of the light-emission surface of the Fresnel lens and the like. In the lens member described in Patent Literature 3, the light reflection surfaces of the prisms are needed to be set higher in order to totally reflect all the light beams that are incident on the light-incident surface close to the rounded top end portions of the prisms. As a result, the lens thickness is disadvantageously increased.
Further, in each of the lenses described in the above Patent Literatures 1 through 3, a loss occurs as part of light which is incident on the light-incident surfaces of the prisms fails to reach the light reflection surfaces of the prisms. This disadvantageously makes it difficult to maximize utilization efficiency of the light emitted from the light source. For example, in Patent Literature 3, between the convex refractive lens portion centered around the center axis and the reflector portion positioned outside the refractive lens portion, there is an area where incident light does not reach the light reflection surface of the reflector portion. The light which transmits this area is lost.
Moreover, radiation light from an LED used as a light source has a luminous intensity distribution in which light intensity is smaller as a light radiation angle is larger. As illustrated in FIG. 6, when the light-emission surface of an LED used as a light source 2 is disposed so as to face the light-incident surface of a conventional total internal reflection (TIR) lens 1, the light incident on the light-incident surface, which is an inner concave lens surface 3 of the TIR lens 1 disposed so as to face the light-emission surface of the LED 2, is totally reflected by a light reflection surface of an outer convex lens surface 4. When the light with relatively high light intensity is incident as incident light L2 from the periphery of a central part of the LED 2 onto the light-incident surface of the TIR lens 1 at a position close to a center axis AX, the incident light L2 is reflected by a portion of the light reflection surface which is on the upper peripheral side of the convex lens surface 4. As a result, the TIR lens 1 has a high luminous intensity in the vicinity of the center axis AX but has a low luminous intensity in the vicinity of an intermediate area outside the vicinity of the center axis and also has a high luminous intensity outside the vicinity of the intermediate area. Accordingly, if the TIR lens 1 is formed into a Fresnel lens by a conventional technology without modification, the emitted light forms ring-shaped unnecessary light (lens flare) around the optical axis as seen in the case of the TIR lens 1, which still causes a problem of deteriorated appearance. Further, in the lens described in Patent Literature 3, both the light-incident surface and the light-emission surface of the reflecting lens portion are aspherical, which causes difficulty in processing to form the lens into a Fresnel lens and increases processing costs.
As a solution to these problems, a lens member has been proposed in which a light-incident surface of a lens disposed so as to face a light source is divided into a plurality of concentric segmented areas centered around the optical axis of the light source. The light-incident surface has a Fresnel lens including a plurality of prism portions which are different in refraction angle corresponding to the segmented areas. The TIR lens is configured to have a concave lens surface which is provided on the lower side of the TIR lens with the center axis as a center, the concave lens surface being disposed so as to face the light-emission surface of the light source. The TIR lens is configured to also have a convex lens surface provided on a peripheral side surface of the TIR lens. Light incident on the lower concave lens surface is totally reflected by the convex lens surface, so that the light travels toward the light-emission surface of the TIR lens. The light-emission surface of the TIR lens is positioned on the upper side of the TIR lens, and is in parallel with the light-emission surface of the light source and also positioned above the light-emission surface of the light source. This TIR lens is formed into a Fresnel lens by dividing the lens into a plurality of prisms. Each of the prisms is composed of a prism incident surface corresponding to a segmented area of the concave lens surface, and a prism reflection surface corresponding to a segmented area of the convex lens surface which totally reflects the light incident on the segmented area. When the prisms divided from the TIR lens are disposed as a Fresnel lens, a prism corresponding to a more upper outside segmented area of the convex lens surface in the TIR lens is disposed at a more inside position of the Fresnel lens (position closer to the center axis). A prism corresponding to a more lower outside segmented area of the convex lens surface in the TIR lens is disposed at a more outside position of the Fresnel lens (a more outer peripheral region of the Fresnel lens). A lens member having such configuration has been proposed (see Patent Literatures 4 and 5).
In this lens member, the above-described division arrangement is adopted when the lens surface of the TIR lens is divided into a plurality of prisms to use the TIR lens as a Fresnel lens. Accordingly, the light at the periphery of the central part with relatively high light intensity is incident on the light-incident surface of the prisms positioned close to the center axis, and is totally reflected by the light reflection surface of the prisms. Therefore, intense light which used to be emitted from the outer side of the conventional TIR lens can be emitted from the vicinity of an intermediate area on the inner side. This makes it possible to obtain a brightness distribution of the light source which looks natural for human eyes as its luminous intensity gradually decreases from a bright center toward a dark outer side. As a result, generation of ring-shaped lens flare which looks unnatural can be suppressed, and its appearance can be improved thereby. Moreover, since the light-incident surface and the light reflection surface which correspond to each other constitute one prism via a ridgeline and the plurality of prisms are disposed in sequence, the light incident on the light-incident surface in each of the prisms is all totally reflected at the moment when the light reaches the adjacent light reflection surface. According to the above configuration, using this lens member as a light source makes it possible to obtain illuminating devices with considerably enhanced light utilization efficiency.